The present invention relates to an apparatus and method for the reduction and oxidation of constituents of a fluid. The invention is particularly applicable to the removal of pollutants from gases.
Removal of nitrogen oxide(s), sulfur dioxide, hydrocarbons and carbon monoxide from motor vehicle exhaust gases and from other fluid streams in various industrial processes presents a significant challenge.
It is critical that the level of these compounds in the environment be minimized. Yet, the processes generating these compounds are vital to the world's economy and the well-being of its people.
As a result, many different means have been employed to minimize these compounds in fluid streams. These involve both means to minimize the production of the compounds and means to remove the compounds after their production.
Most of these means add significantly to the cost and/or complexity of the underlying process. As a result, significant time, effort and resources have been expended to discover better means to minimize the amounts of these compounds released into the environment.
U.S. Pat. No. 4,253,925 to Mason sets forth an apparatus or cell for the reduction of nitrogen oxide(s) and carbon monoxide in an exhaust gas. An oxygen ion conducting ceramic electrolyte is provided with porous electrodes on opposite sides. Air flows by one electrode and exhaust gas flows by the other electrode. An electrical potential is applied between the electrodes, the exhaust gas electrode being negative with respect to the air electrode. Nitrogen oxide(s) and carbon monoxide in the exhaust gas are reduced to nitrogen and carbon, the oxygen picking up electrons from the negative electrode, negative oxygen ions passing through the ceramic and giving up electrons to the positive electrode, and oxygen passing into the air. The design requires communication with air separate from the exhaust gas. The capacity of the apparatus may be increased by increasing length of the cell or operating several independent cells in parallel (in the sense of gas flow) with each electrode electrically connected by a conductor to the respective electrode of another cell either in electrical parallel or series.
Commercial application typically places constraints on the permissible length of a cell and the use of parallel cells as, illustrated in Mason, substantially increases the complexity of the device.
Mason also shows an embodiment wherein the gases flow along a linear series of the cells, wherein only the opposite electrode of each end cell is connected by a conductor to the electrical potential. The alternating adjacent opposite electrodes of the other cells are then interconnected by conducting members between each cell. In this embodiment communication with air separate from the exhaust gas is still required.
Mason's second embodiment is difficult to construct, because it requires substantial fabrication and attention to sealing.
U.S. Pat. No. 4,659,448 to Gordon disclose a single cell similar to Mason's except that both electrodes are exposed to exhaust gas, rather than one to air. This allows not only the reduction of nitrogen oxide(s) and sulfur dioxide at the negative electrode, but also oxidation of carbon monoxide and hydrocarbons at the positive electrode.
As in Mason's apparatus, the capacity of Gordon's device can not be readily increased. Either the length can be increased or discrete cells can be operated in parallel with all the attendant complexity.